The present invention relates to a device for clamping oil well and water well tubulars and rods, including but not limited to drill pipe, drill collars, well casing, production tubing, sucker rods, pump column pipe, and the like, all of which tubulars, pipes and rods are referred to herein simply as "down-hole tubulars". More particularly, this invention relates to such a clamp which precisely centers down-hole tubulars of varying diameters.
In well drilling and well completion operations it is often necessary to lift and to align lengths of down-hole tubulars precisely. For example, in oil or water well drilling, multiple lengths of drill pipe must often be raised from a horizontal position at or near ground level to a vertical position aligned with the center line of the well. Such lifting and aligning operations require some type of clamp for securely holding the tubular in place as it is lifted. When a pivotably mounted transfer arm is used, this clamp must support large loads in several different orientations.
Compounding the problem is the fact that each joint or length of down-hole tubular must be closely aligned with a string of such tubulars after it has been lifted to the vertical position, as when a string of drill pipe or casing is being made up, for example. A clamp for such purposes should preferably provide, without any adjustment, the necessary alignment for down-hole tubulars having various diameters. Proper alignment has been a problem for many such clamps of the prior art, particularly those employing pivoted clamping jaws.
When pivoted clamping jaws are used, there is a tendency for the center of the clamped down-hole tubular to vary as a function of the diameter of the tubular being clamped. This problem may be alleviated somewhat by using guided jaws in conjunction with symmetrically moving pivoted rocker arms. Such an arrangement is shown in a machine tool clamp described by Lorenz in U.S. Pat. No. 3,386,726. In the Lorenz clamp the guided jaws are free to translate with respect to the pivotably mounted rocker arms as the clamp closes.
This approach, however, suffers from the disadvantage that loads are not symmetrically distributed in the clamp for the full range of clamp positions. As the guided jaw translates with respect to the rocker arm, the center of clamping force on the jaw moves.